A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, such as hydrogen or an alcohol such as methanol or ethanol, is supplied to the anode and an oxidant, such as oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.
The principal component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrochemical reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. The platinum or platinum alloy catalyst can be in the form of unsupported nanometre sized particles (such as metal blacks or other unsupported particulate metal powders) or can be deposited as even higher surface area particles onto a conductive carbon substrate, or other conductive material (a supported catalyst).
The MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of an ion-conducting membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst coated ion-conducting membrane. Subsequently, gas diffusion layers are applied to both faces of the catalyst coated ion-conducting membrane. Finally, an MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs, removing products, providing electrical connections and providing physical support.
High electrochemical potentials can occur in a number of real-life operational situations and in certain circumstances can cause damage to the catalyst layer/electrode structure. Further descriptions of a number of situations where high electrochemical potentials are seen are described below:
(a) Cell Reversal
Electrochemical cells occasionally are subjected to a voltage reversal condition, which is a situation where the cell is forced to the opposite polarity. Fuel cells in series are potentially subject to these unwanted voltage reversals, such as when one of the cells is forced to the opposite polarity by the other cells in the series. In fuel cell stacks, this can occur when a cell is unable to produce, from the desired fuel cell reactions, the current being forced through it by the rest of the cells. Groups of cells within a stack can also undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array. Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses reliability concerns. Undesirable electrochemical reactions may occur, which may detrimentally affect fuel cell components. Component degradation reduces the reliability and performance of the fuel cell, and in turn, its associated stack and array.
A number of approaches have been utilised to address the problem of voltage reversal, for example employing diodes capable of carrying the current across each individual fuel cell or monitoring the voltage of each individual cell and shutting down an affected cell if a low voltage is detected. However, given that stacks typically employ numerous fuel cells, such approaches can be quite complex and expensive to implement.
Alternatively, other conditions associated with voltage reversal may be monitored instead, and appropriate corrective action can be taken if reversal conditions are detected. For instance, a specially constructed sensor cell may be employed that is more sensitive than other fuel cells in the stack to certain conditions leading to voltage reversal (for example, fuel starvation of the stack). Thus, instead of monitoring every cell in a stack, only the sensor cell need be monitored and used to prevent widespread cell voltage reversal under such conditions. However, other conditions leading to voltage reversal may exist that a sensor cell cannot detect (for example, a defective individual cell in the stack). Another approach is to employ exhaust gas monitors that detect voltage reversal by detecting the presence of or abnormal amounts of species in an exhaust gas of a fuel cell stack that originate from reactions that occur during reversal. While exhaust gas monitors can detect a reversal condition occurring within any cell in a stack and they may suggest the cause of reversal, such monitors do not identify specific problem cells and they do not generally provide any warning of an impending voltage reversal.
Instead of, or in combination with the preceding, a passive approach may be preferred such that, in the event that reversal does occur, the fuel cells are either more tolerant to the reversal or are controlled in such a way that degradation of any critical cell components is reduced. A passive approach may be particularly preferred if the conditions leading to reversal are temporary. If the cells can be made more tolerant to voltage reversal, it may not be necessary to detect for reversal and/or shut down the fuel cell system during a temporary reversal period. Thus, one method that has been identified for increasing tolerance to cell reversal is to employ a catalyst that is more resistant to oxidative corrosion than conventional catalysts (see WO01/059859).
A second method that has been identified for increasing tolerance to cell reversal is to incorporate an additional or second catalyst composition at the anode for purposes of electrolysing water (see WO01/15247). During voltage reversal, electrochemical reactions may occur that result in the degradation of certain components in the affected fuel cell. Depending on the reason for the voltage reversal, there can be a significant rise in the absolute potential of the fuel cell anode to a higher potential than that of the cathode. This occurs, for instance, when there is an inadequate supply of fuel (i.e. fuel starvation) to the anode. In this situation the cathode reaction and thus the cathode potential remains unchanged as the oxygen reduction reaction (ORR):½O2+2H++2e−→H2Owhereas the normal fuel cell reaction at the anode—the hydrogen oxidation reaction (HOR):H2→2H++2e−can no longer be sustained and other electrochemical reactions then take place at the anode to maintain the current. These reactions can typically be either water electrolysis—the oxygen evolution reaction (OER):H2O→½O2+2H++2e−or carbon electrochemical oxidation:½C+H2O→½CO2+2H++2e−Both these reactions occur at a higher absolute potential than the oxygen reduction reaction at the cathode (hence the cell voltage reverses).
During such a reversal in a PEM fuel cell, water present at the anode enables the electrolysis reaction to proceed and the carbon support materials used to support the anode catalyst and other cell components enables the carbon oxidation reaction also to proceed. It is much more preferable to have water electrolysis occur rather than the carbon oxidation reaction. When water electrolysis reactions at the anode cannot consume the current forced through the cell, the rate of oxidation of the carbonaceous anode components increases, thereby tending to irreversibly degrade certain anode components at a greater rate. Thus, by incorporating a catalyst composition that promotes the electrolysis of water, more of the current forced through the cell may be consumed in the electrolysis of water than in the oxidative corrosion of anode components, such as carbon.
A reversal condition can also be experienced due to oxidant starvation on the cathode. However, this is much less detrimental to the cell, because the reaction likely to occur instead of the reduction of the oxidant is that the protons produced at the anode cross the electrolyte and combine with electrons directly at the cathode to produce hydrogen via the hydrogen evolution reaction (HER):2H+→2e−+H2 In this reversal situation the anode reaction and thus the anode potential remain unchanged, but the absolute potential of the cathode drops to below that of the anode (hence the cell voltage reverses). These reactions do not involve potentials and reactions at which significant component degradation is caused.
(b) Start-up Shut-down
For many fuel cells it is also not practical or economic to provide purging of hydrogen from the anode gas space with an inert gas such as nitrogen during shut down. This means that there may arise a mixed composition of hydrogen and air on the anode whilst air is present on the cathode. Similarly, when a cell is re-started after being idle for some time, air may have displaced hydrogen from the anode and as hydrogen is re-introduced to the anode, again a mixed air/hydrogen composition will exist whilst air is present at the cathode. Under these circumstances an internal cell can exist, as described by Tang et al (Journal of Power Sources 158 (2006) 1306-1312), which leads to high potentials on the cathode. The high potentials can cause carbon to oxidise according to the electrochemical carbon oxidation reaction indicated previously:½C+H2O→½CO2+2H++2e−and this is highly damaging to the structure of the catalyst layer where the catalyst layer contains carbon. If the cathode layer is able to support oxygen evolution by the water electrolysis reaction (OER) however, the high potentials can be used to drive water electrolysis rather than carbon corrosion.
(c) Regenerative Fuel Cells
In regenerative fuel cells the electrodes are bi-functional and both electrodes must support two electrochemical reaction types at different times. When operating as a fuel cell the oxygen electrode must reduce oxygen (ORR) and the hydrogen electrode must oxidise hydrogen (HOR); when operating as an electrolyser the hydrogen electrode must evolve hydrogen (HER) and the oxygen electrode must evolve oxygen (OER).
Electrocatalysts for the water electrolysis reaction are generally based on iridium oxide or iridium oxide mixed with at least one other metal oxide. However, iridium-based catalysts are not sufficiently active at the loadings required in a fuel cell.
It is therefore an object of the present invention to provide a catalyst layer comprising alternative water electrolysis catalysts, which have superior activity to state of the art water electrolysis catalysts for the oxygen evolution reaction, and which demonstrate superior performance when incorporated into a MEA and operated under practical real-life fuel cell operating conditions.